The present invention relates to energetic materials, particularly to nanostructured energetic materials produced by sol-gel chemistry, and more particularly to a method which involves producing a metal-oxide skeletal structure from hydrolyzable salts, and utilizing this skeletal structure to produce metal-oxide-based energetic materials using sol-gel processing. The nanostructured metal-oxide serves as a reactant, specifically an oxidizer, in the energetic material. During gelation the metal-oxide medium also serves as a processing aid in which other energetic material ingredients may be suspended. The metal-oxide also provides mechanical strength to the final energetic material. The invention of making metal-oxide-based energetic materials using sol-gel chemistry is applicable to oxides formed from the following elements of the periodic table: Groups 2 through 13, part of Group 14 (germanium, tin, lead), part of Group 15 (antimony, bismuth), part of Group 16 (polonium), and the lanthanides and actinides.
Metal-oxide-based energetic materials are generally prepared by the mixing of fine powders. An example of which would be pyrotechnics such as thermite, which produce high temperature.
Energetic materials are herein defined as any material which stores chemical energy in a fixed volume. Explosives, propellants, and pyrotechnics are examples of energetic materials. Reaction is initiation by mechanical or thermal stimulus. Explosives and propellants may be thought of as a means of storing gas. Pyrotechnics typically release much of their energy as heat.
Energetic materials consist of fuels and oxidizers which are intimately mixed. This is done by incorporating fuels and oxidizers within one molecule or through chemical and physical mixtures of separate fuel and oxidizer ingredients. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc.
Traditional manufacturing of energetic materials involves processing granular solids into parts. Materials may be pressed or cast to shape. Performance properties are strongly dependent on particle size distribution, surface area of the constituents, and void volume. In many cases achieving fast energy release rates, as well as insensitivity to unintended initiation, necessitates the use of small particles (≦100 μm) which are intimately mixed. Reproducibility in performance is adversely affected by the difficulties of synthesizing and processing materials with the same particle morphology and distribution uniformity. Manufacturing these granular substances into complex shapes is often difficult due to limitations in processing highly solid filled materials.
An example of an existing limitation of processing granular solids is in manufacturing energetic materials for detonators. The state-of-the-art now requires the precise synthesis and recrystallization of explosive powders. These powders typically have high surface areas (e.g., >1 m2/g). The powders are weighed and compacted at high pressures to make pellets. Handling fine grain powders is very difficult.
Dimensional and mechanical tolerances may be very poor as the pellets may contain little or no binder. Changes in the density and dimensions of the pellets affect both initiation and detonation properties. Manufacturing rates are also low as the process is usually done one at a time. Certification of material is typically done by expensive, end-use detonation performance testing and not solely by chemical and physical characterization of the explosive powder. As these detonators or initiating explosives are sensitive, machining to shape pressed pellets is typically not done.
Another current limitation is producing precise intimate mixtures of fuels and oxidizers. The energy release rates of energetic materials are determined by the overall chemical reaction rate. Monomolecular energetic materials have the highest power as the energy release rates are primarily determined by intramolecular reactions. However, energy densities can be significantly higher in composite energetic materials. Reaction rates (power) in these systems are typically controlled by mass transport rates of reactants.
In general, initiation and detonation properties of energetic materials are dramatically affected by their microstructural properties. It is generally known in material science that the mechanical, acoustic, electronic, and optical properties are significantly and favorably altered in materials called “nanostructures,” which are made from nanometer-scale building blocks. Modern technology, through sol-gel chemistry, provides an approach to control structures at the nanometer scale, thus enabling the formation of new energetic materials, generally having improved, exceptional, or entirely new properties.
Since the invention of black powder the technology for making solid energetic materials has remained either the physical mixing of solid oxidizers and fuels, referred to as composite energetic materials (e.g., black powder); or the incorporation of oxidizing and fuel moieties into one molecule, referred to as monomolecular energetic materials (e.g., trinitrotoluene, TNT).
The basic distinctions between these prior known energetic composites and energetic materials made from monomolecular approaches are as follows. In composite systems, desired energy properties can be attained through readily varied ratios of oxidizer and fuels. A complete balance between the oxidizer and fuel may be reached to maximize energy density. Current composite energetic materials can store energy as densely as >23 kJ/cm3. However, due to the granular nature of composite energetic materials, reaction kinetics are typically controlled by the mass transport rates between reactants. Hence, although composites may have extreme energy densities, the release rate of that energy is below that which may be attained in a chemical kinetics controlled process.
In monomolecular energetic materials the rate of energy release is primarily controlled by chemical kinetics, not by mass transport. Therefore, monomulecular materials can have much greater power than composite energetic materials. A major limitation with these monomolecular energetic materials is the total energy density achievable. Currently, the highest energy density for monomolecular materials is approximately 12 kJ/cm3, about half that achievable in composite systems. The reason for this is that the requirement for a chemically stable material and the current state of the art synthetic procedures limit both the oxidizer-fuel balance and the physical density of the material.
A new synthesis approach to forming energetic materials, specifically pyrotechnics, explosives, and propellants, using the chemical sol-gel methodology is described and claimed in U.S. application Ser. No. 08/926,357 filed Sep. 9, 1997, entitled “Sol-Gel Manufactured Energetic Materials” and in International Application No. PCT/US98/18262 (WO 99/12870) published Mar. 18, 1999. In energetic composites we can control oxidizer-fuel balances at the nanometer scale. Sol-gel chemistry involves the reactions of chemicals in solution to produce nanometer-sized primary particles, called “sols.” The “sols” can be linked to form a three-dimensional solid network, called a “gel,” with the remaining solution residing within open pores. Solution chemistry determines the resulting nanostructure and composition, which in turn determine the material properties. Controlled evaporation of the liquid phase results in a dense porous solid, “xerogel.” Supercritical extraction (SCE) eliminates the surface tension and in so doing the capillary forces of the retreating liquid phase that collapse the pores. The results of SCE are highly porous, lightweight solids called “aerogels.” A typical gel structure is characteristically very uniform because the particles and the pores between them are on the nanometer size scale. Such homogeneity ensures uniformity of the material properties, which is one of the key reasons for synthesizing energetic materials using the sol-gel methodology.
A main interest in the sol-gel approach to energetic materials is that it offers the possibility to precisely control the composition and morphology of the solid at the nanometer scale, a result that is difficult or not possible to achieve by conventional techniques. We believe that such control of the nanostructure could enable the creation of entirely new energetic materials with desirable properties. One important result from this work that supports the aforementioned thesis is a decrease in the impact sensitivity of sol-gel derived energetic nanocomposite materials. For example, we have shown that energetic ingredients processed by conventional methods exhibited drop-hammer impact sensitivities <10 cm, whereas the same constituents processed with sol-gel chemistry showed >130 cm sensitivity. This finding with its implications to the safety of energetic materials may be important in and of itself, albeit much more characterization work is required.
In addition to providing fine nanostructural and compositional control, sol-gel methodology offers other advantages to safety in energetic material processing. For example, ambient temperature gelation and low temperature drying schemes prevent degradation of the energetic molecules, and the water-like viscosity of the sol before gelation, allows easy casting to near-net shapes, which may be preferred to the alternatives.
Although sol-gel chemistry is a well-known means of producing nanostructured materials, the above-referenced patent application resulted from the first use thereof to create energetic materials using sol-gel processing. In development of the energetic materials by sol-gel processing, four new classes of energetic materials have been derived, and which comprise: 1) an energetic nanocomposite, 2) nanocrystalline materials, 3) powder addition energetic materials, and 4) energetic skeletal materials, and these four classes of sol-gel produced energetic materials are described hereinafter and illustrated.
Nanocomposites are multicomponent materials in which at least one of the component phases has one or more dimensions (length, width, or thickness) in the nanometer size range, usually defined as 1 to 100 nm. Energetic nanocomposites are a class of materials that have a fuel component and an oxidizer component intimately mixed on the nanometer scale with at least one of the component phases which meets the size definition. An example would be an organic skeletal matrix acting as a fuel, with an oxidizer embedded within this network.
Using a sol-gel procedure first described by R. W. Pekala, J. Mater. Sci. 24, 3221 (1989), to make aerogels, a porous organic solid matrix containing —CH2—, was prepared by the polycondensation of resorcinol with formaldehyde (RF). Subsequent crystallization of an oxidizer, ammonium perchlorate (AP), within the pores of the gel matrix, completes the synthesis. The oxidizer was first dissolved in water, then added to the reacting RF sol prior to gelation. Gelation of the RF sol occurred first, then crystallization of the AP was induced by exchanging the liquid residing in the pores for a solvent in which the oxidizer was insoluble. The final step of removing the pore fluid was done by either a slow evaporation resulting in a dense solid, or by supercritical extraction (SCE) with carbon dioxide, leading to a low density, highly porous material.
Transmission electron microscopy (TEM) of the nanocomposite showed a solid structure composed of interconnected clusters of nanometer size primary particles, and crystallites, generally smaller than 20 nm. Near-edge x-ray absorption microscopy of the same sample showed that nitrogen was uniformly distributed in the material on a scale less than 43 nm, which is the limit of resolution for that technique. These results for the nanostructure were consistent with other published reports on the structure of sol-gel derived materials.
Small-angle neutron scattering (SANS), a non-destructive method for characterizing nanostructures, was conducted at the National Institute of Standards Technology on both the energetic nanocomposites and the fuel-only samples. The result for the fuel-only (RF) specimens is consistent with other SANS data for gels, which typically give average primary particle sizes of a few nanometers. By comparison, the results for the RF-AP nanocomposite bulk material and thin films, show extremely small (≈1 nm) particles (probably oxidizer crystals that have grown within the gel pores), as well as >10 nm particles that are presumed to be larger oxidizer crystals that have grown to occupy several gel pores. Sol-gel chemistry ultimately offers a means to control the distribution and size of crystals within the gel matrix.
Differential scanning calorimetry (DSC) is a method commonly used to establish the internal energy content of energetic materials by measuring exothermic energy releases as a function of temperature. DSC plots for a RF-AP nanocomposite and neat ammonium perchlorate were compared. The trace for the RF-AP nanocomposite showed an exotherm at about 260° C., indicating that it is indeed energetic, whereas the trace for the neat AP showed no exothermic reaction in the absence of the fuel skeleton (RF).
The present invention follows the above-described sol-gel processing approach and involves metal-oxide-based energetic. In the energetic material of the present invention, a sol-gel synthesized metal-oxide oxidizer reacts rapidly with a fuel distributed in the gel exchanging the oxygen and releasing significant amounts of energy. The invention involves methods to make metal-oxide-based energetic materials. When processed with fuels e.g., particulate metals, polymers, etc., nanostructured energetic materials are fabricated. The invention involves fabricating a metal-oxide skeletal structure prepared from hydrolyzable metal salts (i.e., metal salts or metal alkoxides), after which a fuel (e.g., metal or other inorganic or organic fuels) is added to the sol prior to gelation or grown within the solid matrix after drying.